Field of the Invention
The present invention relates generally to an anti-skid brake control system for an automotive vehicle. More specifically, the invention relates to a technology for preventing wheel slippage from causing abrupt and substantial variation.
Japanese Patent Second (allowed) Publication (Tokko) Showa 50-34185, Japanes Patent First (unexamined) Publication (Tokkai) Showa 56-79043 disclose typical construction of anti-skid brake control systems. Each of the disclosed system includes means for detecting wheel speed and projected vehicle body speed data, means for deriving a wheel slippage and wheel acceleration and means for comparing brake control parameters, such as the wheel speed, wheel slippage and wheel acceleration, with respectively corresponding reference values for controlling braking pressure in a wheel cylinder.
In such anti-skid brake control, wheel slippage varies in relation to variation of braking force or braking torque with a certain lag factor. This lag factor sometimes influences for performance of anti-skid control. Namely, assuming the braking torque is TQB, a wheel slippage is .lambda., a wheel acceleration is .alpha., inertia moment on the wheel is I, an angular velocity of the wheel is .omega., a road surface friction coefficient is .mu., a weight load applied for the wheel is W and an effective radius of the wheel is R, a vehicle body speed, K is a proportional constant, and in a micro period, V is constant the equation of motion of the wheel can be as follows: ##EQU1##
From the foregoing equation (1), the wheel acceleration .alpha. can be illustrated by: ##EQU2##
Assuming .DELTA.V=V-R.omega. and thus .omega.=(1/R).times.(V-.DELTA.V), the foregoing equation can be modified as: EQU I.times.(d/dt).times.{V-.lambda.)/R}=K(.lambda./V)WR-TQB
By Laplace transformation of the foregoing equation, the following equation can be obtained: EQU I{-(.lambda./R)s}=K(.lambda./V)WR-TQB
Therefore, wheel slippage can be derived by: EQU .lambda.(s)=(V/KWR)/{1+(IV/KWR.sup.2)s}.times.TQB (3)
While the braking torque TQB is increased, wheel slippage increases with a certain lag time. When held constant at the increased level after increasing braking torque, the wheel slippage is maintained to increase for the presence of the certain lag time as the primary lag factor as can be appreciated from the foregoing equation (3). On the other hand, as can be clear from the foregoing equation (2), the wheel aceleration .alpha. varies according to variation of the wheel slippage.lambda. when the braking torque TQB is maintained constant. Accordingly, when after switching the operational mode in the skid control cycle from an APPLICATION mode for increasing the braking torque to a HOLD mode for holding the braking torque constant, the wheel deceleration .alpha. decreases according to increasing of the wheel slippage .lambda.. After decreasing the magnitude wheel deceleration across a predetermined deceleration criterion and decreasing of wheel slippage across a predetermined wheel slippage criterion, the operational mode again returns to the APPLICATION mode to increase the braking pressure. Since the wheel slippage variation contains a certain primary lag factor relative to variation of the braking torque variation, braking torque can be excessively increased during this lag period to cause expansion of the period to maintain RELEASE mode for decreasing the braking pressure for recovery of wheel traction. This causes excessive braking torque to be applied to the vehicular wheel to cause reduction of the road friction coefficient and whereby to cause cornering force. This may degrade driving stability of the vehicle.